In order to implement quantum computation, five conditions proposed by DiVincenzo should be necessarily satisfied regardless of a physical quantity used as a unit of a material or information used for the quantum computation. First, a quantum bit, which is a basic unit of the quantum computation, should be clearly defined and can be integrated. Second, a state of the quantum can be initialized to a well-known state. Third, a coherence time of the quantum bit should be long. Fourth, a general-purpose gate should be provided for a single quantum bit and two quantum bits. Fifth, a state of the quantum bit should be measurable.
The above five conditions are factors that should be necessarily satisfied in implementing practical quantum computation, and if any one of the conditions cannot be satisfied, the practical quantum computation cannot be implemented.
Until now, a lot of methods and materials for implementing the quantum computation have been proposed. Although there are some cases in which quantum computation of a basic level is succeeded, not a case has ever been succeeded in satisfying the five conditions of DiVincenzo.
It is proved that NMR (Nuclear Magnetic Resonance) and Ion Trap, which have been most actively studied in the early stage, cannot integrate the quantum bits to a level capable of implementing the practical quantum computation. In addition, quantum dot devices using a semiconductor are currently spotlighted as a solution for the integration problem.
However, a GaAs semiconductor which is a representative quantum dot device using a semiconductor has a problem in that a spin state of electrons is easily broken due to the noises generated by nuclear spin. In addition, it is impossible to accurately control and measure a state of integrated quantum dots based on an existing structure of connecting the quantum dots in series. Accordingly, required is a quantum dot device and a method of manufacturing thereof, which can satisfy all the five conditions of DiVincenzo described above and accurately measure and control a state of each of the integrated quantum dots while the spin state of electrons is not easily broken by the nuclear spin.